I think the demonic energy flowing through her body is simply wearing it out. I think one of the reasons she drinks so much is to feed that energy, to keep it at bay. But it just "fuels the fire" and creates illnesses all its own - and that burning energy just continues to burn, day after day after day.
She's played with that fire her whole life... and treated it as her slave.
It doesn't like her.
I think you are on to something.